Can I re-export an animation?
Never ever render directly to video... (and avoid AVI as a container  much as you can). There is no way to break down the composited image into its original parts any more, and on top of that a video encoded and image will likely yield errors due to compression.

Use a low resolution/low quality video format just to evaluate the pacing of the animation, but do the final render as an image sequence. That will allow you to avoid situations like the one you are facing where you will have to re-render to make any changes.

If you want to do compositing later, save as image sequences to a format like OpenEXR, in which you can save not only the final rendered image but proper Alpha channel, Z depth information, object and material IDs and whatever other render layer information you want.

Then you can composite as you like and do a final video encoding later.

(read Save all render passes to a single EXR image)